The present invention concerns a coaxial cable with at least one internal conductor and a cable shield surrounding the at least one internal conductor. It also concerns a testing method for such a coaxial cable.
Coaxial cables are generally known. The normally comprise a single internal conductor and a cable shield concentrically surrounding the internal conductor. However, coaxial cables are also already known that comprise a plurality of internal conductors arranged concentrically relative to one another, for example, the “Clogston conductor” (see, for example, FIG. 3 of U.S. Pat. No. 2,841,792). Such coaxial cables are also considered to be coaxial cables in the sense of the present invention.
Coaxial cables are used in, among other things, magnetic resonance systems in order to connect “local coils” with a control and evaluation device. The local coils are spatially arranged such that they can detect magnetic resonances that were excited immediately prior in an examination subject via a radio-frequency excitation field with an excitation frequency. The excitation frequency normally corresponds to the Larmor frequency of the appertaining magnetic resonance system.
During the excitation of the magnetic resonances, the cable shield is likewise exposed to the excitation field. Sheath waves that oscillate with the excitation frequency can thereby be excited on the cable shield.
In the prior art, it is known to arrange a plurality of sheath wave barriers along the cable shield. Each sheath wave barrier comprises two ends, whereby the ends are coupled to the cable shield at coupling points of the cable shield that are separated from one another. The sheath waves that are excited on the cable shield due to the excitation field can be damped via the sheath wave barriers.
The coupling of the sheath wave barriers to the cable shield can be direct and permanent (see, for example, U.S. Pat. No. 5,294,886 German Patent Document No. 10 2004 015 856). However, the sheath wave barriers can also be shunted to the cable shield and possibly even be displaceable along the cable shield in operation (see, for example, U.S. Pat. No. 6,822,846). Both types of the coupling are also possible in the scope of the present invention.
Like any other component, sheath wave barriers can also fail. When a sheath wave barrier fails, the danger exists that a usable signal that should be transferred over the at least one internal conductor is corrupted or otherwise impaired. Local high voltage levels and/or high currents can also be induced on the cable shield. The examination subject can be harmed by this. Consequential damages (for example, to the cable shield, to the local coil or to the control and evaluation device of the magnetic resonance system) can also result.